M193 still used by military? Yes (with caveats).
Other posters have already explained valid differences in performance caused by differences in bullet weights (M855 "Green Tip" 62 grain vs. M193 Vietnam era 55 grain) and rifling twists (1:14, 1:12, 1:9, and 1:7).
The preponderant DoD Standard ammo for current issue M16A2s, A3s, A4s, M4s, M4A1s, and M249 SAW light machineguns is M855 Ball. It works best out of the 1:7 twist barrels equipping those currently issued military weapons. It will not stabilize properly out of older M16A1 rifles (with 1:12 twist) past about 80-90 yards. Attempting a 300 yard shot would be an exercise in futility. It will however suffice for close in emergency shooting (under 75 yards in my experience) although you aren't going to get tight shot groups. Past 100 yards and you start shooting a widely dispersed "shotgun" pattern using rifle bullets. The dispersion increases with range to the target.
The Substitute Standard is still M193 Ball (of which there are still warehouse loads although rapidly becoming scarce in some venues). This ammo was manufactured and stockpiled by the millions for the original M16A1 & XM-177s (CAR-15s) of 1960s-1970s production vintage. It has been given away or sold by the boatload to allies whom we have equipped with older M16A1s. It continues to be manufactured domestically and by numerous overseas manufacturers. M193 will function out of newer military weapons with 1:7 rifling although accuracy suffers a bit. It makes great live fire CQB training ammo (which is what we use it for). It can be requested for training (at least in the Army), but is rarely seen downrange except for a very few Reserve or Guard units still equipped with M16A1s (and who are often issued M855 anyway out of simple ignorance on the part of unit logisticians).
The Army authorizes M193 for use in a "combat emergency" (i.e., that's all ya got left; no more M855).
Most civilian ARs of recent (90's - 00's) manufacture have 1:9 twist rates and can accurately shoot either bullet weight although the weapon must be zeroed for one or the other.
